Asphalt compositions suitable for coating roofing shingles are generally produced by selecting a suitable asphalt, or asphalt flux, and processing the asphalt to obtain particular properties useful for a coating asphalt. For instance, it is important that the asphalt in roofing products retain some degree of hardness and do not flow under conditions of high temperature. Such an increased hardness is generally accompanied by a reduced penetration level, an increased viscosity, and an increased softening point.
One way in which the asphalt may be processed consists of air blowing, which is an oxidation process that involves blowing air through molten asphalt to modify the physical properties of the asphalt. In such an air blowing process, air is blown through an asphalt flux for a particular period of time at an elevated temperature. The air blowing asphalts process generally raises the softening point of the asphalt, which increases the shingles' ability to resist flowing at high temperatures on a roof, lowers the shingle penetration into a range that allows proper press of granules without becoming too brittle, raises their melt viscosity such that when filler is added, the filled coating viscosity is in a range that allows the roofing shingle to process at high speeds, and creates a shingle that will perform over many years on the roof in spite of being exposed to sun, high temperatures, and inclement weather conditions.
Conventional asphalt compositions used for manufacturing roofing shingles incorporate a special “coating grade” asphalt as the feedstock to the air blowing process in order to meet the desired properties for a roofing application. Such coating grade asphalts often comprise materials that are naturally softer (higher penetration, lower viscosity) than paving grade asphalts, which are considered “hard” asphalts.
U.S. Pat. No. 7,951,239, the disclosure of which is incorporated herein by reference in its entirety, discloses a method for producing coating asphalts from paving grade asphalt feedstocks. The method includes partially air blowing the paving grade asphalt feedstock to lower its penetration value and raise its softening point. A wax is then added to the partially blown asphalt to further raise its softening point to a point that is within the desired range for a coating asphalt. This process additionally incorporates a blowing catalyst, such as phosphoric acid to further soften the asphalt.